Hey.

So my 2005 S2000 has a missfire problem. I have the codes; po300, po301, po302, po303, and po304.
Ill explain the symptoms and whats been checked.
The symptoms are is that when the car is first started from a cold start there is no problem, but after about 5 or more seconds the missfires start; you can feel them, and also the CEL will come on; but even after a CEL reset, the cel wont turn on right away, but takes a little bit.
The missfires will only happen at idle; driveability of the car is still what its suppose to be in normal or hard driving.

One thing to note is that when hooked up to a a obd2 scanner. The freeze frame data shows
Short term fuel trim (SHRTFT1(%)) is at 15.6% at idle
Long term fuel trim (LONGFT1(%)) is at 15.6% at idle
because the fuel trim is so high at idle; it would normally signal a vacuum leak; but I did check for vacuum leaks and could find none with a smoker.
Spark plugs, and coils replaced
Injectors cleaned and work properly
leak down and compression test was done; they showed it was everything was fine.
what could really be causing the car to be running so rich if its not a vacuum leak.

other numbers from freeze frame data if it helps
MAP(PSI) 4.6
RPM 811
spark advance 4.0
IAT (degrees F) 123
TP% 5.5
02B1S1(V) .055
02B1S2(V) .115
SHRTFTB1S1 (%) 13.3
SHRTFTB1S2 (%) 13.3
LOAD PCT (%) 30.2
ECT (degrees F) 183

 

My old AP1 used to do the same damn thing...only misfire at idle. Replaced plugs, injectors, cleaned IAC valve and nothing happened. Eventually took it to a place locally that specializes in Honda and Acura and they told me I had burnt valves and would cost $2500 to fix. Needless to say, I didn't go that route. It ran and drove fine aside from the misfire when i was at a red light or at a stop sign. The CEL was intermittent. For months at a time it would go away by its self...then come back for a few days, same codes as you, (the P0300 is a multi cylinder misfire IIRC) then go away.

I could never find the solution to mine...it may have been burnt valves. By the time I called to inspect the car prior to pick up, it was buttoned up and the shop didn't take any pictures like I requested to confirm the burnt valves.

edit: a second, more performance style shop, did a compression and leak down test and everything was normal IIRC. Maybe start w/ this?

 

Your description is the same as whats happening to my car.
I already have done a leak down and a compression. and they are fine.

 

May be something with the electrical system, no dash flickering, computer Maybe?

Have you done a valve adjustment?

 

Do a valve adjustment. After I did a valve adjustment on mine, the misfire was still there, but came and went. Just like the CEL so I don't think it was the valves.